Electrical power transformers are used primarily as voltage transformation devices wherein the voltage input is either transformed up to a higher level or down to a lower level. In the distribution of electrical energy it is customary to transform the voltage up to a higher level at the generating source, distribute the energy via transmission lines at the high level and then transform the voltage back to a lower level for use by the load. This is an efficient means of transmitting electrical energy over long distances. For short distances, the transformer is used to adapt loads designed for different voltages to the local distribution system and to provide voltage separation for safety reasons.
Power transformers generally consist of insulated copper wire wound on an iron core and, in its simplest form, consists of two windings usually referred to as a primary winding and a secondary winding. The turns ratio on these two windings establish the voltage step up or down characteristics of the device. At very low power levels, a single winding is sometimes used in a configuration referred to as an auto transformer. In some of these devices, the input/output voltage ratio is varied by a sliding contactor thereby making electromechanical contact with non-insulated portions of the single winding.
The electrical power transformer has been the backbone of the electric utility distribution system for many years. The power transformer has a high efficiency and a good history of reliability. However, in these days of energy consciousness and environmental restrictions, all aspects of electrical hardware engineering from power distribution through distribution to the end user are under scrutiny.
Iron core and copper coils are heavy, bulky and relatively expensive to manufacture. Because such transformers first transform electrical energy to magnetic energy which, in turn, is transferred back to electrical energy, some energy is dissipated in this procedure.
Prior art solid-state transformers do not require an iron core and copper coils. They are composed of integrated circuits, capacitors and inductors. Such a transformer transforms electrical energy directly from input to output in electrical form so that its energy efficiency is relatively high. Even though integrated circuits in a solid state transformer do consume some energy, such energy consumption is relatively small.
One example of such a solid-state transformer is disclosed in the U.S. Pat. No. 4,347,474, to Brooks et al. The Brooks '474 patent discloses a solid-state regulated power transformer with pulse-width modulation. An AC input signal is chopped in a solid state switching converter at a frequency substantially higher than the frequency of the input signal and then filtered to attenuate the high-frequency components while passing the frequency of the AC input signal. A feedback signal modulates the duty cycle of the switching converter to provide automatic voltage regulation under varying loads and leading and lagging power factors.
The Loen U.S. Pat. No. 4,598,349 discloses a high-frequency electronic transformer comprising flyback: converters. An input capacitor of one embodiment keeps the high-frequency voltage harmonics produced by the switching operation remote from the mains. The input capacitor has relatively low impedance at the switching frequency and a relatively high impedance at the output frequency.
The Das U.S. Pat. No. 4,866,585 discloses an AC to DC high-frequency switching power supply.
The Peters et al. U.S. Pat. No. 4,408,268 discloses an AC duty cycle modulated voltage controller.
The Olla U.S. Pat. No. 4,302,717 discloses a switched AC pulse width modulator power supply.
Soviet Union Patent Document No. 488,197 discloses a pulse width modulated switch mode AC power supply.
Other patents of a more general interest include U.S. Pat. No. 3,517,297 to Durio et al, McMurray U.S. Pat. No. 3,538,417, Derby U.S. Pat. No. 3,735,237, Yokoyama U.S. Pat. No. 4,321,662, Okuyama et al U.S. Pat. No. 4,328,454, Shima et al U.S. Pat. No. 4,361,866 and Schutten et al U.S. Pat. No. 4,706,183.
One problem associated with such prior art solid state transformers is that while such transformers provide the voltage transformation function, such transformers do not provide a separation function. For example, in the '349 Patent noted above, at FIGS. 2A-2D, S4 and S3 yield a direct path between input CD and output EF.
With respect to the '474 Patent noted above, there is a direct path between input point 12 and output point 34, since both are grounded.
One attempt to solve this separation function problem of the prior art is by specifying the way to plug the input to the power source. However, this approach is not practical since not all power sockets are marked with polarity and, if a fuse on the grounding wire is blown (a 50% probability if a fuse on the circuit is blown), the whole transformer circuit will have a relatively high voltage potential (110 volts in the U.S.) which is beyond the recommended safety value (i.e. 35 volts) for human beings.